Turbo-compressors are devices that employ a rotational drive to blades to compress a fluid (typically a gas, such as air). Examples include centrifugal compressors, in which the blades are part of an impeller, although the principles described herein may be applied other types of turbo-compressor as well. One common use of turbo-compressors is in turbochargers. Strict limits imposed by legislative bodies are setting great challenges for engineers across the industry to develop greener means of vehicle propulsion. In order to cut down carbon dioxide emissions and address legislative limits, engine downsizing and turbocharging methods are trendsetting technologies that tackle these challenges and maintain engine performance. Engine downsizing involves reducing the swept volume of the engine, which compromises the performance of the engine, while forced induction boosting methods are adopted to satisfy driveability requirements.
The single-stage turbocharger is one of the typical boosting configurations adopted for a heavily downsized engine to ensure driveability requirements are met. However, the single-stage turbocharger is limited by the compressor flow range. This limitation produces a poor low-end engine torque response. Often, engine designers limit the pressure they specify from the turbocharger and therefore the degree of engine downsizing in order to achieve an acceptable low-end engine torque response. The poor low-end torque response arises because the compressor flow range is restricted by its choke and surge limits. The choke flow of the impeller is where the relative fluid Mach number in the impeller is equal to one—i.e. the relative flow speed reaches the speed of sound. This occurs at higher mass flow rates. At lower mass-flow rates, the flow is subjected to fluid separation and flow reversal. This causes the compressor to stall and surge. Surge results in a reduction in compressor efficiency and a drop in pressure ratio, which damages the mechanical integrity of the compressor and shortens compressor life.
Various measures are being taken by engineers to design compressors that suppress surge. These solutions try to tackle surge and allow stable compressor flow operation across a wider flow range. One commonly used method known as Self-Recirculating Casing Treatment (SRCT) relies on the dynamics of the flow to recirculate mass back into the compressor inlet. The different types of SRCT optimisation methods can be broken down into three main elements: slot location, symmetric, and asymmetric. Slot location looks at adjusting the location of the slots for the recirculation path and optimising it to enhance the surge characteristics of the compressor. SRCT optimisation is also investigated by looking at the channel shape of the recirculating air flow path—i.e. symmetric and asymmetric channel shapes. Examples of these SRCT methods can be found in: published US Patent application no.: 2012/0308371 A1; Tamaki Hideaki, Unno Masaru, Kawakubo Tomoki, Hirata Yukuta. Aerodynamic Design of Centrifugal Compressor for AT14 Turbocharger. IHI Engineering Review. Vol: 2 No. 2. 2010; and U.S. Pat. No. 6,945,748 B2.
The three main elements of SRCT referred to above essentially improve the compressor flow operating range by enhancing the surge characteristics of the centrifugal compressor. However, there are other means of tackling surge that do not involve making design changes to the compressor housing. Instead a recirculation kit/valve is used to recirculate compressed fluid back to the impeller inlet. These conventional compressor recirculation methods involve bleeding off fluid after the compressor outlet using a valve, typically actuated either pneumatically or electrically. These have been adopted by researchers to stabilise the flow in surge and, in certain scenarios, to aid other elements of the engine architecture such as exhaust gas recirculation (EGR). However, conventional recirculation is generally used as a transitional or temporary measure to avoid over-pressure and surge during tip-out conditions. Tip-out occurs when a large amplitude pressure wave is generated in the engine's intake piping, as in the event of the throttle closing. A pressure spike is caused by a column of air in the pipe decelerating suddenly due to sudden blockage at the throttle and the compressor not allowing pressure dispersion at the other end of the pipe. Without a compressor recirculation valve, the flow is subjected to reversal and inefficient or destructive dissipation of energy. This behaviour is cyclic and occurs at high frequencies, which also causes unacceptable noise levels (see, for example: Ali Ghanbanriannaeeni, Ghazalehsandat Ghazanfari Hashemi. Protecting a Centrifugal Compressor from Surge. Pipeline and Gas Journal. Vol. 240. No. 4. 2012; or What is the Recirculation Valve? [Online] Available from: http://www.turbomaster.info/eng/turbos/recirculation.php [Accessed Apr. 4, 2013].)
Another approach that has been used to tackle surge is the use of active or passive systems to modify the compressor's inlet pipe geometry, often called “pre-swirl devices”. These achieve better part-load compressor performance and extend the operating range of a compressor by offsetting compressor “surge”. However, this is usually at the cost of a reduced full-load performance. FIG. 1 illustrates the principle, showing a pair of compressor blades 10, which are aligned at an angle α to the (axial) direction of the incoming flow as shown by arrow A. The blades 10 are moving in the direction of arrow B. The resultant velocity of the air relative to the blades is shown by arrow C. Introducing a component of swirl to the flow, as shown by arrow D produces a resultant relative velocity shown by arrow E. If the angle of velocity E is equal to the angle of the blades, a, then the flow runs smoothly along the blades 10 and the compressor operates with maximum efficiency. Without swirl (relative velocity C), the flow separates from the backs of the blades 10. This causes the fluid to recirculate along the backs of the blades 10 to fill the low-pressure zones left behind by the flow separation. When this happens on a macroscopic scale, the compressor is experiencing surge.
Normal pre-swirl devices allow the surge margin to be improved by directing flow along the angle of the compressor blades. However, this is achieved at a cost of shifting the choke mass flow rate to a lower value. Several researchers have proposed various different pre-swirl configurations which involve the use of various blade or inlet guide vane (IGV) arrangements that may be fixed (i.e. stators) or adjusted (i.e. rotors). Hiroshi et al conducted a study looking at compressor map width enhancement (MWE) (15) using a synergistic approach of combining variable inlet-guide vanes and compressor casing treatment as shown in FIG. 1 (See: Hiroshi Uchida, Akinobo Kashimoto, Yuji Iwakiri. Development of a Wide Flow Range Compressor with Variable Inlet Guide Vane. R&D Review of Toyota CRDL. Vol. 41. No. 3; and Map Width Enhancement (MWE) [Online] available from [accessed Aug. 29, 2013]: http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1647215.
FIG. 2 shows the integration of a pre-swirl device, referred to as a variable inlet guide vane (VIGV) 20, in an inlet duct 22, with a SRCT 24 in a modified compressor casing 26, and an impeller 28. The results of a Computational Fluid Dymanics (CFD) simulation of this geometry are shown in FIG. 3, which shows both the efficiency (upper graph) and pressure ratio (lower graph) as a function of air flow rate for the conventional compressor, for the compressor with only the SRCT 22, and for the compressor with a VIGV angle of 70 degrees at a range of compressor speeds N of 0.38 N0, 0.63 N0, 0.88 N0 and N0 the design speed. Also shown are “surge lines” for VIGV angles of 60 and 80 degrees. The surge lines are lines linking the points of minimum mass flow rate at which the pressure ratio can be achieved without surge. This may also be referred to as the surge margin for a particular compressor configuration. The results showed that installation of the SRCT alone resulted in an improvement of 30% of the surge margin relative to the conventional (i.e. unmodified) compressor at a pressure ratio of 2.5.
It can also be seen from FIG. 3 that the surge margin for the integrated configuration decreases as the VIGV angle increases. The authors claim that the surge margin flow rate can be reduced by 59% relative to the conventional compressor at a pressure ratio of 2.5 by changing the VIGV angle. Hiroshi et al also conducted an experimental study to investigate the benefits of the VIGV device alone (i.e. without any SRCT recirculation). The results showed that the reduction in surge margin was less than 15%. Thus, the authors suggested the synergistic configuration is more optimal to achieve a wide flow range centrifugal compressor. On the other hand, FIG. 3 also shows the penalty on the choke flow limit of the compressor (for the 70 degree VIGV) which is significantly reduced. This is the cost many pre-swirl devices incur in order to improve the surge flow margin of the centrifugal compressor. In terms of efficiency, as shown in FIG. 3 even though a slight improvement is seen in the compressor efficiency at low flow rates, the peak efficiency of the compressor is compromised by approximately 10%. This is due to the pressure losses that occur across the VIGV device as the vane angle increases at higher mass flow rates.
An IGV/VIGV improves stability at lower mass flow rates and reduces the volume of recirculating air required to achieve certain high-boost, low-flow operating conditions. However, the use of IGVs results in the compressor choking at higher engine speeds and causes a significant reduction of peak compressor efficiency.
Conventionally, compressors are coupled to a drive using a bolt which extends all the way through the hub. This leaves the end of the bolt exposed to the inlet fluid flow. As the end of the bolt will typically include a hexagonal head for tightening/releasing the bolt, this presents a surface which creates a drag on the incoming fluid and has an adverse effect on performance.
The present invention has been conceived with the aforementioned problems in mind.